Radionuclides may be produced by bombarding a target with an accelerated particle beam as may be generated by a cyclotron, linear accelerator, or the like. The target contains a small amount of target material that is typically provided in the liquid phase but could also be a solid or gas. The target material includes a precursor component that is synthesized to the desired radionuclide in reaction to irradiation by the particle beam. As but one example, F-18 ions may be produced by bombarding a target containing water enriched with the 0-18 isotope with a proton beam. After bombardment, the as-synthesized F-18 ions may be recovered from the water after removing the water from the target. The production of F-18 ions in particular has important radiopharmaceutical applications. For instance, the as-produced F-18 ions may be utilized to produce the radioactive sugar fluorodeoxyglucose (2-fluoro-2-deoxy-D-glucose, or FDG), which is utilized in positron emission tomography (PET) scanning. PET is utilized in nuclear medicine as a metabolic imaging modality in the diagnosis of cancer.
The production of radionuclides such as F-18 ions is an expensive process, and thus any improvement to the production efficiency and yield would be desirable. Unfortunately, the application of the particle beam initiates the desired nuclear reaction in only a very small fraction of the radionuclide precursors in the target. The particle beam deposits a significant amount of heat into the target material residing in the target during bombardment. For instance, in the conventional production of F-18 ions, it has been found that only about one of every 2,000 protons stopping in the target water actually produces the desired nuclear reaction, with the rest of the proton beam merely depositing heat. Yet the amount of radioactive product that can be produced in a radionuclide target is proportional to the amount of heat that can be removed during bombardment of the target material of choice. The heat energy deposited in the target material may cause boiling and generate bubbles or voids in the volume of target material. Bubbles or voids do not yield radionuclides; the particle beam simply passes through the bubbles or voids to the back of the target structure. Moreover, the rapidly increasing vapor pressure developed in the target chamber containing the target material as a result of the heat deposition may cause the target to structurally fail if the heat deposition is not adequately removed.
Radionuclide production yield could be increased by increasing the beam energy inputted to the target, but due to the foregoing problems the beam energy has been intentionally limited in conventional systems. Conventional radionuclide production systems may provide a means for cooling the beam targets generally by routing a heat transfer medium such as water to the target to carry heat away therefrom during bombardment. Conventional target designs, however, do not have sufficient capacity for heat removal, and as a result the radionuclide production yield and efficiency has been less than desirable in conventional targets.
In view of the foregoing, there is an ongoing need for beam targets utilized for radionuclide production that enable increased capacity and efficiency for removing heat and thus improved radionuclide production yield and efficiency.